Selenium toxicity is thought to involve the incorporation of selenoamino acids into proteins. The protein synthesizing machinery of selenium-tolerant organisms, such as the selenium accumulating Astragali, are believed unable to recognize selenocystein and selenomethionine, resulting in the exclusion of these selenoamino acids from proteins. Cell-free protein synthesizing systems will be prepared with polysomes and post-polysomal fractions from selenium accumulator species to determine whether selenoamino acids are excluded from proteins; similar cell-free systems will be prepared from non-accumulator species. Heterologous systems will be used to study the site of exclusion of the selenoamino acids along the protein synthesizing sequence. The possibiliy of a specific pathway for selenium assimilation existing in those bacteria which possess selenoenzymes, will be investigated by studying the transport and metabolism of selenate, selenite, selenocysteine and selenomethionine by Escherichia coli. Techniques will be developed for the identification of selenol compounds through derivatizations with N-ethylmaleimide or by carboxymethylation. These techniques will allow a direct examination of the presence or absence of selenocysteine in the proteins of accumulator species, and will assist in the identification of metabolites of selenium compounds in bacteria.